This invention relates to a feedback control system for the control of the rate of a flow of a gas which is either drawn from or introduced into an exhaust passage of an internal combustion engine with the purpose of reducing the concentrations of pollutants in the exhaust gas. A control system of the invention can be embodied both in an exhaust gas recirculation control system and in a secondary air control system.
In internal combustion engines and particularly in automotive engines, it is one of prevailing methods for lessening nitrogen oxides (NOx) in the exhaust gas to recirculate a portion of the exhaust gas from an exhaust passage back into the combustion chambers so as to lower the maximum combustion temperature thereby to suppress the formation of NOx in the combustion chambers. The exhaust gas recirculation (EGR) is carried out by the use of a flow control valve for controlling the quantity of the recirculated exhaust gas relative to the quantity of fresh air or air-fuel mixture admitted into the combustion chambers in dependence on the operating condition of the engine.
For lessening unburned hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas, a widely employed technique is the injection of secondary air into the exhaust line of the engine to oxidize HC and CO before emission into the atmosphere. In many cases this technique is practised with the provision of either a thermal reactor or a catalytic converter in the exhaust line. The air injection is carried out by the use of a flow control valve designed so as to regulate the quantity of the secondary air in dependence on the engine operating condition.
Either in conventional EGR systems or in conventional secondary air injection (SAI) systems, it is usual to accomplish an open loop control by taking an intake vacuum at or downstream of a main throttle valve as a primary indication of the engine operating condition and using a vacuum-operated flow control valve, though the type of the control valve is different between an EGR system and a SAI system. Closed loop or feedback control is seldom employed in EGR and SAI systems principally because of difficulty in measuring simply and accurately the flow rate of either actually recirculated exhaust gas or actually introduced secondary air.
In a typical prior art EGR system, a flow control valve comprises a diaphragm which supports a valve member and, as an element of a valve actuator, serves as a wall of a vacuum chamber connected to an induction passage at a section where is located a main throttle valve, and the magnitude of vaccum applied to the valve actuator is corrected according to certain factors of engine operating condition such as the cooling water temperature (by the use of a temperature-sensitive stop valve) and the exhaust pressure (by the use of an air admission valve of a diaphragm type). Since no feedback is made and the control is accomplished wholly mechanically, it is impossible in this EGR system to continuously vary the quantity of the recirculated exhaust gas relative to the fresh air in response to changes in the engine operating condition and it is difficult to enlarge the degree of freedom of the control as desired without rendering the system unsuitable for practical use for automotive engines by reasons of complexity in mechanism, rise of cost and lowering of reliability. Besides, the quantity of the recirculated exhaust gas itself exhibits some deviation from an intended value because of limitations to the precision of the control valve and other components. It has been difficult, therefore, to maintain an optimum proportion of the recirculated exhaust gas to the fresh air over a wide range of the engine operating condition, inevitably resulting in that the effectuation of EGR adversely influences the fuel economy, driveability and output characteristic of the engine.
A typical prior art SAI system comprises an air pump driven by the engine for passing secondary air admitted through an air inlet, which is independent of a main induction passage, to an exhaust passage through a secondary air duct which is equipped with a check valve downstream of the air pump and branches at a section between the pump and the check valve to a vacuum-operated control relief valve. A vacuum transmission passage connects the control relief valve to the induction passage at a section, for example, downstream of the main throttle valve so that a variable amount of air supplied from the air pump is discharged into the atmosphere through the control relief valve in dependence on the magnitude of intake vacuum, whereby the quantity of secondary air introduced into the exhaust passage is controlled. In this system too, the control is accomplished wholly mechanically, meaning a relatively small degree of freedom of the control, without making any feedback. It is very difficult, therefore, to realize the introduction of an optimum quantity of secondary air in accordance with changing operating condition of the engine. For example, the extent of the control is such that the air supplied from the air pump is entirely injected into the exhaust passage during medium speed medium load operation of the engine but is partially discharged through the control relief valve while the engine is operated under different conditions at higher speeds, at higher loads or at lower loads. Accordingly the quantity of the secondary air tends to become either so excessive as to render the exhaust gas temperature insufficient for completing the intended oxidation reactions or too small to supply a required quantity of oxygen, resulting in that the lessening of HC and CO is not always achieved to satisfaction and that, as an additional disadvantage, there occurs a considerable increase in fuel consumption.